RFID (Radio Frequency IDentification) tags need to include security functions, yet at the same time their resources are extremely limited. Moreover, to provide privacy, authentication and protection against tracking of RFID tags without loosing the system scalability, a public-key based approach is inevitable. In this paper, we present an architecture of a state-of-the-art processor for RFID tags with an Elliptic Curve (EC) processor over GF(2163 ). It shows the plausibility of meeting both security and efficiency requirements even in a passive RFID tag. The proposed processor is able to perform EC scalar multiplications as well as general modular arithmetic (additions and multiplications) which are needed for the cryptographic protocols. As we work with large numbers, the register file is the most critical component in the architecture. By combining several techniques, we are able to reduce the number of registers from 9 to 6 in the EC processor. To obtain an efficient modulo arithmet...